An example of blood pressure calculation method used by an electronic sphygmomanometer is an oscillometric method. In the oscillometric method, a cuff including a fluid bladder wrapped around a portion of a living body is pressurized and depressurized. A volume change of the fluid bladder transmitted by a volume change of a pressurized blood vessel is recognized as a pressure change of the fluid bladder (pressurized pulse wave amplitude), and thereby a blood pressure is calculated.
The fluid bladder has such property that the pressure of the fluid bladder and the volume of the fluid bladder have a relationship as shown in FIG. 33. More specifically, the description will be made with reference to FIG. 33. In a region in which the pressure of the fluid bladder is low as shown in portion A, the volume of the fluid bladder rapidly increases in response to an increase of the volume of the fluid bladder. On the other hand, as shown in portion B, as the pressure of the fluid bladder increases, an increasing rate of the volume of the fluid bladder in response to an increase of the pressure of the fluid bladder gradually decreases.
An electronic sphygmomanometer for measuring a blood pressure during depressurizing process of a fluid bladder will be described. At this occasion, FIG. 34 illustrates a case where a fluid density in a fluid bladder is low, and FIG. 35 illustrates a case where a fluid density in a fluid bladder is high. More specifically, FIGS. 34 and 35 illustrate a volume change of the fluid bladder (portion (B)), a change of a fluid density in the fluid bladder (portion (C)), and a pressure change of the fluid bladder (portion (D)) according to a volume change of a blood vessel (portion (A)). Further, FIG. 36 illustrates a case where a discharge rate of the fluid discharged from the fluid bladder is fast, namely, the amount of discharge per unit time is large, and FIG. 37 illustrates a case where a discharge rate of the fluid discharged from the fluid bladder is slow, namely, the amount of discharge per unit time is small. More specifically, FIGS. 36 and 37 illustrate a volume change of the fluid bladder (portion (B)) and a pressure change of the fluid bladder (portion (C)) according to a volume change of a blood vessel (portion (A)).
It is understood from FIGS. 34 to 37 that the electronic sphygmomanometer for measuring the blood pressure during depressurizing process of the fluid bladder has the following features in detection accuracy of a volume change of the blood vessel.
(1) As the pressure of the fluid bladder is higher, the density of the fluid in the fluid bladder is higher.
(2) As the volume of the fluid bladder is larger, a density change of the fluid in the fluid bladder caused by a volume change of the blood vessel is smaller. Accordingly, detection accuracy of the volume change of the blood vessel is low.
(3) In a case where the volume change of the fluid bladder is the same, detection accuracy changes as follows. As the pressure of the fluid bladder is higher, a density change of the fluid in the fluid bladder caused by the volume change of the fluid bladder is larger, and accordingly detection accuracy of the volume change of the blood vessel is high.
(4) Even when the pressure of the fluid bladder is the same, a magnitude of the volume change of the fluid bladder caused by a volume change of the blood vessel changes according to the amount of discharge of the fluid in the fluid bladder. Accordingly, detection accuracy of the volume change of the blood vessel is different.
(5) As the amount of discharge of the fluid in the fluid bladder is larger, the volume change of the fluid bladder caused by the volume change of the blood vessel is smaller. Accordingly, the detection accuracy of the volume change of the blood vessel becomes low.
Therefore, in the electronic sphygmomanometer for using the oscillometric method to measure the blood pressure during depressurizing process of the fluid bladder, the detection accuracy of the volume change of the blood vessel relies on the density of the fluid in the fluid bladder and the amount of discharge of the fluid discharged from the fluid bladder.
In the electronic sphygmomanometer for reducing the pressure of the fluid bladder at a constant rate and measuring the blood pressure during depressurizing process, the pressure is reduced at a constant rate as shown in FIG. 38A. Therefore, the amount of fluid discharged from the fluid bladder is controlled by a valve according to the pressure of the fluid bladder and a perimeter of a measurement portion as shown in FIG. 38B. As a result, as shown in FIG. 38C, in a region in which the pressure of the fluid bladder is high, a pressurized pulse wave amplitude with respect to a constant volume change of the blood vessel is large, and in a region in which the pressure of the fluid bladder is low, a pressurized pulse wave amplitude with respect to a constant volume change of the blood vessel is small. On the other hand, the amount of change of the volume change of the blood vessel caused by the pressure change of the fluid bladder is different according to a perimeter of a measurement portion. Therefore, these are the causes of errors in blood pressure measurement.
Subsequently, an electronic sphygmomanometer for measuring a blood pressure during pressurizing process of a fluid bladder will be described. At this occasion, FIG. 39 illustrates a case where a fluid density in a fluid bladder is low, and FIG. 40 illustrates a case where a fluid density in a fluid bladder is high. More specifically, FIGS. 39 and 40 illustrate a volume change of the fluid bladder (portion (B)), a change of a fluid density in the fluid bladder (portion (C)), and a pressure change of the fluid bladder (portion (D)) according to a volume change of a blood vessel (portion (A)). Further, FIG. 41 illustrates a case where an inflow of a fluid into a fluid bladder is fast, i.e., the amount of inflow per unit time is large, and FIG. 42 illustrates a case where an inflow of a fluid into a fluid bladder is slow, i.e., the amount of inflow per unit time is small. More specifically, FIGS. 41 and 42 illustrate a volume change of the fluid bladder (portion (B)) and a pressure change of the fluid bladder (portion (C)) according to a volume change of a blood vessel (portion (A)).
It is understood from FIGS. 39 to 42 that the electronic sphygmomanometer for measuring the blood pressure during pressurizing process of the fluid bladder has the following features in detection accuracy of a volume change of the blood vessel.
(1) As the pressure of the fluid bladder is higher, the fluid density in the fluid bladder is higher.
(2) As the volume of the fluid bladder is larger, a change of a fluid density in the fluid bladder caused by a volume change of the fluid bladder is smaller. Accordingly, detection accuracy of the volume change of the blood vessel is low.
(3) In a case where the volume change of the fluid bladder is the same, detection accuracy changes as follows. As the pressure of the fluid bladder is higher, a fluid density change in the fluid bladder caused by the volume change of the fluid bladder is larger, and accordingly detection accuracy of the volume change of the blood vessel is high.
(4) Even when the pressure of the fluid bladder is the same, a magnitude of the volume change of the fluid bladder caused by a volume change of the blood vessel changes according to the amount of inflow of the fluid in the fluid bladder. Accordingly, detection accuracy of the volume change of the blood vessel is different.
(5) As the amount of inflow of the fluid into the fluid bladder is larger, the volume change of the fluid bladder caused by the volume change of the blood vessel is smaller. Accordingly, the detection accuracy of the volume change of the blood vessel becomes low.
Therefore, in the electronic sphygmomanometer for using the oscillometric method to measure the blood pressure during pressurizing process of the fluid bladder, the detection accuracy of the volume change of the blood vessel relies on the density of the fluid in the fluid bladder and the amount of inflow of the fluid into the fluid bladder.
In the electronic sphygmomanometer for increasing the pressure of the fluid bladder at a constant rate and measuring the blood pressure during pressurizing process, the pressure is increased at a constant rate as shown in FIG. 43A. Therefore, the amount of fluid flowing into the fluid bladder is controlled by a pump according to the pressurizing rate of the fluid bladder and a perimeter of a measurement portion. At this occasion, the amount of fluid flowing into the fluid bladder changes according to the pressure of the fluid bladder and the perimeter of the measurement portion as shown in FIG. 43B. As a result, as shown in FIG. 43C, in a region in which the pressure of the fluid bladder is high, a pressurized pulse wave amplitude with respect to a volume change of the blood vessel is large, and in a region in which the pressure of the fluid bladder is low, a pressurized pulse wave amplitude with respect to a constant volume change of the blood vessel is small. On the other hand, the amount of change of the pressurized pulse wave amplitude caused by the pressure change of the fluid bladder is different according to the perimeter of the measurement portion. Therefore, these are the causes of errors in blood pressure measurement.
In an electronic sphygmomanometer for pressurizing a fluid bladder by keeping a constant drive voltage of a pump for pressurizing the fluid bladder, the pressurizing rate of the fluid bladder changes according to the pressure of the fluid bladder and a perimeter of a measurement portion as shown in FIG. 44A. In addition, as shown in FIG. 44B, the amount of fluid flowing into the fluid bladder changes according to the pressure of the fluid bladder. As a result, as shown in FIG. 44C, in a region in which the pressure of the fluid bladder is high, a pressurized pulse wave amplitude with respect to a constant volume change of the blood vessel is large, and in a region in which the pressure of the fluid bladder is low, a pressurized pulse wave amplitude with respect to a constant volume change of the blood vessel is small. On the other hand, the amount of change of the volume change of the blood vessel caused by the pressure change of the fluid bladder is different according to a perimeter of a measurement portion. Therefore, these are the causes of errors in blood pressure measurement.
Japanese Unexamined Patent Publication No. H6-245911 (hereinafter, Document 1) discloses a technique for adjusting the amount of discharge of a valve according to a perimeter of a measurement portion or a technique using a fluid storage unit in communication with a fluid bladder and performing control for keeping a constant summation of volumes of the fluid bladder and the fluid storage unit according to a winding perimeter of the fluid bladder to a measurement portion. Therefore, even when the perimeter of the measurement portion is different, the depressurizing rate is kept constant.
Further, Japanese Unexamined Patent Publication No. H5-329113 (hereinafter, Document 2) discloses a method previously arranging a volume change property of a fluid bladder with respect to a pressure of the fluid bladder, converting a signal of the pressure change of the fluid bladder into a volume change, and measuring a blood pressure value using the volume change.
Further, Japanese Unexamined Patent Publication No. H4-250133 (hereinafter, Document 3) discloses a method for closing a valve for discharging a fluid in a fluid bladder in a pulse wave appearance period to prevent attenuation of a volume change of a blood vessel caused by a volume change of the fluid bladder.
Patent Document 1: Japanese Unexamined Patent Publication No. H6-245911
Patent Document 2: Japanese Unexamined Patent Publication No. H5-329113
Patent Document 3: Japanese Unexamined Patent Publication No. H4-250133